Aromatic polycarbonate resins are well known in the art and have long been used alone and as components in molding compositions. Also known in the art are polycarbonate based compositions which include impact modifying agents. The incorporation of a relatively small amount of an elastomer which has a low glass transition temperature is known to improve the impact performance of the resin. Thus, the additions of any of EPDM, SBS and SEBS block copolymers, MBS, ABS, polyolefins and acrylate based interpolymeric composites to polycarbonate resins have been reported in the patent literature.
While the elastomeric additives generally provide improved impact performance, the beneficial effect which thus characterize some such compositions do not endure long exposure to heat. Polycarbonate compositions which are imparted improved impact performance by the incorporation of butadiene-rich graft copolymers such as ABS or MBS therewith are notorious for their poor heat aging characteristics. Furthermore, polycarbonate compositions which contain rubbery impact modifiers often exhibit lower flowability and/or poor cosmetic properties. The term cosmetic properties in the present context includes such known effects as pearlescent whitening, color streaking and blushing. The present invention is therefore directed to the technical solution to these problems.
Blends of polycarbonate and ABS are well known and are commercially available, for instance under the trademark Bayblend from Mobay Corporation. Noted among the patents relating to this technology are U.S. Pat. Nos. 3,130,177 and 3,852,393. Similar compositions were disclosed in U.S. Pat. Nos. 3,954,905 and 3,988,389.
Polycarbonate compositions containing olefinic resinous components have been disclosed in U.S. Pat. Nos. 3,801,673; 4,122,131; 4,205,141; 4,410,662; and 4,420,584.
A tough thermoplastic polycarbonate composition was disclosed in U.S. Pat. No. 4,172,859. The composition contains a copolymer the generic description of which embraces ethylene copolymers, including ethylene-propylene. Maleic anhydride is listed as a possible comonomer and grafting as a method of preparation is mentioned. U.S. Pat. No. 4,677,162 disclosed a composition containing a polycarbonate resin, an ABS graft copolymer and an impact modifying graft. The composition is said to exhibit low gloss values.
Significantly, the preferred grafted olefinic copolymer of the present invention is known for its heat aging properties and for its utility as an impact modifier for thermoplastic resins, including polycarbonate. On the other hand, the copolymer is incompatible with and forms only a dispersion in polycarbonate resins. Furthermore, the preferred grafted olefinic copolymer does not color nor dye well by coloring agents which are normally suitable for use in polycarbonates. This leads to unfavorable cosmetics in molded parts.